Prior Technologies use a variable Intermediate Frequency (IF) signal frequencies, and a fixed Local Oscillator (LO) reference signal frequency. The variable IF signal requires a lot of available bandwidth on the Inter-Facility Link (IFL) between the Indoor Unit and Outdoor Radio.
FIG. 1A illustrates a frequency plan used by a prior art VSAT terminal communicating over two inter-facility link (IFL) cables between a modem and radio equipment using linear radio technology. The IFL may transmit signals over a spectrum 112 using a first of the two IFL cables from an indoor modem to an outdoor unit, where the signals include a DC power 100 signal, a low frequency Local Oscillator (LO) reference signal 104 at 10 MHz and an IF reference signal 106 ranging from 950 MHz to 1950 MHz. Usually, the outdoor unit receives signals at higher frequencies from a satellite (around 20 GHz), and down-converts them to L-Band signals 110 in a frequency range of 950 MHz to 2150 MHz. As the IF reference signal 106 utilizes a broad/wide frequency spectrum that overlaps with the received signals 110 from the ODU to the IDU, the first IFL cable cannot be used to transmit signals from the outdoor unit to the indoor modem without significant manipulation of either the IF reference signal 106 or the received signals 110. The prior art VSAT product uses the second of the two IFL cables to send the down-converted received signal to the IDU from the ODU because the frequency overlap of the IF reference signal 106 from the indoor modem and the received signals 110 from the outdoor unit would interfere with one another on a single IFL.
FIG. 1B illustrates a frequency plan used by a prior art VSAT terminal communicating over a single inter-facility link (IFL) from a modem to radio equipment using linear radio technology. The IFL may transmit signals from an indoor modem to an outdoor unit, where the signals include a DC power 100′ signal, a low frequency Local Oscillator (LO) reference signal 104′ at 10 MHz and an IF reference signal 106′ ranging from 2200 MHz to 3000 MHz. Usually, the outdoor unit receives signals at higher frequencies from a satellite (around 20 GHz), and down-converts them to L-Band signals 110′ in a frequency range of 950 MHz to 2150 MHz. With this prior art implementation, a guard band 120′ between the RX signal 110′ (950 to 2150 MHz) and the TX IF 106′ (2200 to 3000 MHz) is very small, i.e., about 50 MHz, and makes the multiplexor design to separate the frequencies in both the IDU and ODU very difficult. Also, the TX IF 106′ (2200 to 3000 MHz) suffers a lot more attenuation over a long cable as compared to signals at or below 2 GHz. Additionally, calibrating signal losses TX IF 106′ (2200 to 3000 MHz) during installation is time consuming and more prone to error than a fixed signal, for example, a 2 GHz signal.
Some companies have developed VSAT products having only a single IFL between a VSAT system modem and outdoor radio equipment. However, the outdoor radio equipment includes expensive custom conversion chips to perform double frequency conversions in order to avoid frequency overlap of transmit and receive signals on the single IFL.